Refractory hard metals (RHM) as a class are hard, dense materials with high melting points, and are generally of low solubility in most reagents and molten cryolite and resistant to corrosive attack by most acids and alkalis.
RHMs have high electrical conductivity due to their metallic structure; consequently, this combination of properties has made them candidates for use as electrodes in molten salt electrolysis processes where their corrosion resistance and conductivity are vital properties needed for economical performance.
The RHMs have other properties which have limited their usage up to the present time. They are usually brittle, have little resistance to thermal shock, and are quite expensive to produce and fabricate into useful articles.
RHM articles have been produced by a number of processes including hot pressing of the granular or powdered materials, chemical vapor deposition, and in situ reduction of metals by carbon or other reducing agents. Hot pressing is the most commonly used process for production of shapes. A die and cavity mold set is filled with powder, heated to about 300.degree.-800.degree. C. and placed under pressure of about 2.times.10.sup.8 Pa, then removed from the mold and heated at about 1500.degree.-2000.degree. C., or higher, or sintered in the mold.
Hot pressing has the limitations of applicability to simple shapes only, erosion of the mold, and slow production. The pieces produced by hot pressing are subject to a high percentage of breakage in handling, making this process expensive in terms of yield of useful products.
The RHMs of most interest include the carbides, borides, and nitrides of the metals of groups IVA, IVB, VB, and VIB of the periodic table, particularly Ti, V, Si, and W. Of these, the borides are of most interest as electrodes in high temperature electrolysis applications due to their electrical conductivity. Of the borides, TiB.sub.2 has been extensively investigated for use as a cathode or cathodic element in the Hall-Heroult cell.
Typically the Hall cell is a shallow vessel, with the floor forming the cathode, the side walls a rammed coke-pitch mixture, and the anode a block suspended in the molten alumina-cryolite bath at an anode-cathode separation of a few centimeters. The anode is typically formed from a pitch-calcined petroleum coke blend, prebaked to form a monolithic block of amorphous carbon. The cathode is typically formed from a pre-baked pitch-calcined anthracite or coke blend, with iron cast-in-place around steel bar electrical conductors in grooves in the bottom side of the cathode.
During operation of the Hall cell, only about 25% of the electricity consumed is used for the actual reduction of alumina to aluminum, with approximately 40% of the current consumed by the voltage drop caused by the resistance of the bath. The anode-cathode spacing is usually about 4-5 cm., and attempts to lower this distance result in an electrical discharge from the cathode to the anode through suspended aluminum droplets.
The aluminum is present as a liquid pad in the cell overlaying the cathode, but is not in a quiescent state due to the factors of preferential wetting of the carbon cathode surface by the cryolite melt in relation to the molten aluminum, causing the aluminum to form droplets, and the erratic movements of the molten aluminum from the strong electromagnetic forces generated by the high current density.
The wetting of a solid surface in contact with two immiscible liquids is a function of the surface free energy of the three surfaces, in which the carbon cathode is a low energy surface and consequently is not readily wet by the liquid aluminum. The angle of a droplet of aluminum at the cryolite-aluminum-carbon junction is governed by the relationship ##EQU1## where .alpha..sub.12, .alpha..sub.13, and .alpha..sub.23 are the surface free energies at the aluminum carbon, cryolite-carbon, and cryolite-aluminum boundaries, respectively.
If the cathode were a high energy surface, such as would occur if it were a ceramic instead of carbon, it would have a higher contact angle and better wettability with the liquid aluminum. This in turn would tend to smooth out the surface of the liquid aluminum pool and lessen the possibility of interelectrode discharge allowing the anode-cathode distance to be lowered and the thermodynamic efficiency of the cell improved, by decreasing the voltage drop through the bath.
Typically, amorphous carbon is a low energy surface, but also is quite durable, lasting for several years duration as a cathode, and relatively inexpensive. However, a cathode or a cathode component such as a TiB.sub.2 stud which has better wettability and would permit closer anode-cathode spacing could improve the thermodynamic efficiency and be very cost-effective.
Several workers in the field have developed refractory high free energy material cathodes. U.S. Pat. No. 4,297,180, Foster, Oct. 27, 1981, discloses a cell for the electrolysis of a metal component in a molten electrolyte using a cathode with refractory hard metal TiB.sub.2 tubular elements protruding into the electrolyte. Ser. No. 043,242, Kaplan et al. (Def. Pub.), filed May 29, 1979, discloses Hall cell bottoms of TiB.sub.2. Ser. No. 287,125, filed July 27, 1981, by L. A. Joo' et al., discloses sintered refractory hard metal objects. U.S. Pat. No. 4,308,114, Das, Dec. 29, 1981, discloses operation of a Hall cell with TiB.sub.2 cathodes. U.S. Pat. No. 4,308,115, Foster, Dec. 29, 1981, discloses a Hall cell cathode of graphite coated with TiB.sub.2. European Appln. No. 81810185.9, Swiss Aluminium, publ. Dec. 2, 1981, discloses TiB.sub.2 cathodic elements for Hall cells.